Field of the Invention
The present invention relates to a control method for a robot apparatus, a computer readable recording medium, and a robot apparatus.
Description of the Related Art
Robots having multi-joint robot arms and end effectors provided at the tips of the robot arms are used in manufacturing lines, where products are manufactured. A joint mechanism of such a robot arm includes a servomotor, such as an AC servomotor or a DC brushless servomotor, and a reduction gear located on the output side of the servomotor so as to obtain a high torque output. The joint mechanism is connected to a structural member or element, such as a link. An angle is detected by a rotary encoder directly connected to the rotating shaft of a motor provided on the input side of the reduction gear (hereinafter referred to as an “encoder”). The position of the tip of the robot arm (the tip of a hand of the robot) is controlled in accordance with a detection result. Since the encoder provided on the input side of this reduction gear does not detect distortion or looseness of the reduction gear, a position error of the tip of the robot arm may occur. In addition, a position error may also occur at the tip of the robot arm when the posture of the robot arm or the mass of a work piece changes. A timing belt or a strain wave gear is often used in a drive system. Distortion or looseness occurs in this type of drive system, which is a cause of a position error of the tip of the robot arm.
In addition, a configuration is known that may reduce position error of the tip of the robot arm. In the configuration, an encoder is provided at an output shaft of the reduction gear. In addition, a robot has been proposed which is equipped with encoders at both an input shaft and an output shaft of a reduction gear and has a high-accuracy mode which uses encoder information of the output shaft and a high-speed mode which does not use encoder information of the output shaft (for example, see Japanese Patent Laid-Open No. 2011-176913). In a numerically controlled apparatus that performs finishing and roughing, fully closed control is performed during finishing and semi-closed control is performed during roughing. In this manner, a configuration has been proposed with which control is switched between semi-closed control and fully closed control in accordance with a desired accuracy (for example, see Japanese Patent Application Laid-Open No. 62-184504).
In this specification, the high-accuracy mode in Japanese Patent Laid-Open No. 2011-176913 is technically regarded as the same as the fully closed control in Japanese Patent Laid-Open No. 62-184504 and the high-accuracy mode is also referred to as fully closed control. In addition, the high-speed mode in Japanese Patent Laid-Open No. 2011-176913 is technically regarded as the same as the semi-closed control in Japanese Patent Laid-Open No. 62-184504 and the high-speed mode is also referred to as semi-closed control.
To perform the fully closed control illustrated in Japanese Patent Laid-Open Nos. 2011-176913, 62-184504, and the like, a mechanism of a reduction gear and the like is included in a control loop and a servo system tends to oscillate because of the mechanism. Thus, a low servo gain needs to be achieved and, as a result, an operation speed tends to be reduced. In contrast, to perform the semi-closed control, the size of a mechanism including a reduction gear and the like is smaller than that for performing the fully closed control in a control loop and the servo system oscillates to a smaller degree. Thus, a high servo gain may be achieved and a high-speed operation is often possible. As described above, in general, when the same mechanism is controlled, fully closed control is often better than semi-closed control in terms of accuracy and semi-closed control is better than fully closed control in terms of operation speed.
In addition, in the case where a task is performed by a robot arm, a plurality of process operations are necessary. In the range that satisfies a user's desired accuracy, if semi-closed control is performed for as many process operations as possible, it is highly likely that an operation time of the robot arm may be reduced. In addition, the robot arm is constituted by a plurality of joints. In the range that satisfies the user's desired accuracy, if semi-closed control is performed on as many joints as possible, it is highly likely that an operation time of the robot arm may be reduced.
The present invention reduces an operation time of a robot apparatus by increasing a percentage of semi-closed control through determination of process operations or joints that need fully closed control in the range that satisfies a user's desired accuracy and through performance of fully closed control only in necessary cases.